Often, the xmldefault.cnf.xml contains the instruction for which firmware the phone should be running. If you are trying to "repack" or upgrade your phone system, and the firmware files listed in the XML are missing from the TFTP directory, the phone will hang indefinitely. Network Access Control (ACLs) & Firewalls

Only use repacks on lab equipment or fully air-gapped networks.

When a Cisco IP phone boots up, it attempts to contact the TFTP server to download its configuration. If the phone displays a message like and gets stuck, reboots, or fails to register, it indicates a specific breakdown in the communication between the phone and the Call Manager (CUCM) or TFTP server.

If repacking fails, force the phone to skip default:

The phone looks for its unique configuration file.